Vanadium Oxide Films Research Articles

Annealing temperatures effect on the electrical and structural properties of nanocrystalline vanadium dioxide films prepared by Sol–Gel technique

Vanadium oxide (VO2) films glass substrate deposition was achieved at room temperature by the sol–gel method. The fabricated VO2 films were annealed for 1 h at 200, 250, and 300 °C in air. The annealing temperature’s effects on the electrical and structural properties of VO2 films were investigated by DC conductivity, dielectric, AC conductivity, and X-ray diffraction (XRD). XRD results of the annealed films reveal the presence of polycrystalline VO2 with a monoclinic structure. Using the Scherrer equation, an average grain size of VO2 films was estimated to be 2.1–10.8 nm. The density of annealed films was found to be 4.53–4.75 g/cm3. The DC-electrical conductivity reveals the semiconductor behavior of all samples. The activation energy of the prepared films was deduced as 0.22–0.38 eV. The DC conductivity for the VO2 films decreases at an annealed temperature higher than 200 °C, which may due to the increase in grain boundary scattering due to the increase in grain size. The dielectric measurements show that the dielectric loss and dielectric constant decline with the increasing frequency however of these parameters with the annealing temperature. Moreover, the increasing of the AC conductivity with the temperature and frequency is observed. These results suggest that the mobility of charge carriers was increased.

Correlation between Raman spectra and oxygen content in amorphous vanadium oxides

Amorphous vanadium oxide films were produced using a cathodic arc deposition technique. Varying oxygen pressure in a chamber during the film growth, we obtained the samples with different oxygen concentration. Elemental composition of our films was studied by Rutherford backscattering spectroscopy and energy dispersive X-ray analysis. Raman characterization of the samples showed the strong influence of oxide stoichiometry on the spectra: the main feature was attributed to stretching vibrations of vanadyl bonds and its position shifted from 879 to 937 cm−1 upon changing the V/O ratio from VO2 to V2O5. An explanation of such behavior and an empirical formula of this dependence were suggested.

Vanadium Oxide‐Modified Triphenylamine‐Based Hole‐Transport Layer for Highly Reproducible and Efficient Inverted Perovskite Solar Cells

Inverted perovskite solar cells (PSCs) often employ triphenylamine‐based materials, such as poly(triarylamine) (PTAA), as hole‐transporting layer (HTL). However, the high hydrophobic PTAA usually induces unsatisfactory perovskite growth, resulting in inferior efficiency and reproducibility. Herein, the vanadium oxide (VOx) film is introduced to modify hydrophobic PTAA layer and establish a hydrophilic surface for high‐quality perovskite film due to the decreased surface tension with improved perovskite solution spread, nucleation, and growth. The PSCs based on PTAA/VOx HTL exhibit high power conversion efficiency up to 18.9% with a sharp increase in comparison to the 16.5% of the control device. Moreover, the reproducibility is also significantly improved, showing a much smaller standard deviation of 1.75 for 40 devices compared to the control devices of 3.64. This work demonstrates that the introduction of VOx is a promising approach in producing high‐quality perovskite films on poor‐wetting PTAA layer for high‐performance inverted PSCs.

Control of Polymorphic Properties of Multivalent Vanadium Oxide Thin Films

In this study, VxOy thin films were deposited on borosilicate glass substrates using direct current (DC) magnetron sputtering. The optoelectronic thermochromic properties of the resulting multiphase vanadium oxide thin films were investigated. As-deposited (at 280 °C) films were annealed at 350, 450, and 500 °C in an oxygen atmosphere for 30 min in a tube furnace to improve the crystallinity. Structural analysis indicated the formation of a mixed-phase vanadium oxide film consisting of VO2(B), V4O9, and V2O5 phases on the amorphous substrate after annealing above 350 °C. The results showed that the semiconductor-to-metallic phase transition temperature of the vanadium oxide film increased from 48 to 63 °C with increasing annealing temperatures. The sample annealed at 450 °C exhibited the highest variation in the infrared (IR) transmittance (ΔTIR = 28.42%) and the resistivity switch decreased by two orders of magnitude (1.4 × 10–1–2.3 × 10–3 Ω/cm). The thermal treatment temperature affected the width of the thermal hysteresis loop (HLW) and slope stiffness. A narrower HLW of 1.9 °C and a sharp slope stiffness of 8.74 were obtained for the sample annealed at 500 °C. The slope stiffness plays an important role in the fabrication of ultrafast tunable energy-saving smart windows and IR switches.

一种高性能氧化钒热敏薄膜的制备和应用

A method for preparing high performance vanadium oxide thermosensitive thin films and its application were reported. Using reactive magnetron sputtering film deposition technology, the preparation process of vanadium oxide thin films was optimized by changing the sputtering power during the deposition of vanadium oxide thermosensitive thin films, the deposition rate of vanadium atoms was adjusted when they touched the surface of the substrate after being sputtered. At the same time, the equipment was modified and upgraded, that is, a control power supply outside the vanadium sputtering chamber was added to accurately control the sputtering voltage and oxygen partial pressure and other parameters to accurately control the current density in the reaction process. A vanadium oxide film with a sheet resistance of 500 kΩ/□ and a temperature coefficient of resistance (TCR) of −2.7% K−1 was prepared. The experimental results show that the noise equivalent temperature difference (NETD) performance of uncooled infrared focal plane detector made of high-performance vanadium oxide thermal sensitive film is reduced by 30% and the noise is reduced by 28%. The overall performance of the uncooled focal plane detector has been improved significantly.

The Effect of Growth Parameters on Electrophysical and Memristive Properties of Vanadium Oxide Thin Films.

We have experimentally studied the influence of pulsed laser deposition parameters on the morphological and electrophysical parameters of vanadium oxide films. It is shown that an increase in the number of laser pulses from 10,000 to 60,000 and an oxygen pressure from 3 × 10−4 Torr to 3 × 10−2 Torr makes it possible to form vanadium oxide films with a thickness from 22.3 ± 4.4 nm to 131.7 ± 14.4 nm, a surface roughness from 7.8 ± 1.1 nm to 37.1 ± 11.2 nm, electron concentration from (0.32 ± 0.07) × 1017 cm−3 to (42.64 ± 4.46) × 1017 cm−3, electron mobility from 0.25 ± 0.03 cm2/(V·s) to 7.12 ± 1.32 cm2/(V·s), and resistivity from 6.32 ± 2.21 Ω·cm to 723.74 ± 89.21 Ω·cm. The regimes at which vanadium oxide films with a thickness of 22.3 ± 4.4 nm, a roughness of 7.8 ± 1.1 nm, and a resistivity of 6.32 ± 2.21 Ω·cm are obtained for their potential use in the fabrication of ReRAM neuromorphic systems. It is shown that a 22.3 ± 4.4 nm thick vanadium oxide film has the bipolar effect of resistive switching. The resistance in the high state was (89.42 ± 32.37) × 106 Ω, the resistance in the low state was equal to (6.34 ± 2.34) × 103 Ω, and the ratio RHRS/RLRS was about 14,104. The results can be used in the manufacture of a new generation of micro- and nanoelectronics elements to create ReRAM of neuromorphic systems based on vanadium oxide thin films.

Phase transition of ultrasonically sprayed VOx thin films: The role of substrate temperature

In this study, vanadium oxide (VOx) semiconductor films were deposited on glass substrates using ultrasonic spray pyrolysis technique. The effect of the substrate temperature on the physical properties of the deposited thin films was investigated. For this purpose, 0.05 M VCl3 aqueous solutions were prepared and sprayed on glass substrates at different temperature of 225 °C, 275 °C, 325 °C and 375 °C. Structural properties of VOx thin films were investigated by taking x-ray diffraction (XRD) patterns and it was determined that the films deposited at 225 °C and 275 °C have tetragonal V4O9 phase while the ones deposited at 325 °C and 375 °C have mixture of α-V2O5 and β-V2O5, α-V2O5 being dominant. The optical band gap energies of the films were determined to be in the range of 2.29 eV to 2.42 eV. Electrical investigations revealed that VOx films have n-type conductivity and electrical resistivity decreased from 4.07 Ωcm to 0.67 Ωcm depending on the increase in substrate temperature. Scanning electron microscopy (SEM) images showed that morphology of the films highly is sensitive to substrate temperature.

Research of Optical Performance of VO2/V2O5 Composite Film with Ultra-High Luminous Transmittance

The optical performance of vanadium dioxide film is important for the application in smart windows. In this work, we prepared ultra-thin vanadium oxide films with thickness from 30 nm to 120 nm and investigated the influence of thickness on the optical performance of ultra-thin vanadium oxide films. The experimental results show that the composite of vanadium pentoxide (V2O5) with a low refractive index can improve luminous transmittance, which reaches 66.32% at the film thickness of 30 nm. The transmittance of metallic phase ultra-thin vanadium oxide film in the visible band is higher than that in the semiconductor phase, which leads to cross-points in transmittance curve and negative solar energy modulation. With the increase of thickness, the position of cross-points in transmittance curve shifts to long-wavelength, and the absolute value of negative solar energy modulation increases from 0.73% to 1.5%, which is related to the principle of optical interference. These conclusions will inspire ultra-thin vanadium dioxide film in improving the solar energy modulation of smart windows under the high luminous transmittance.

Acoustoelectric drag current in vanadium oxide films

Two different Mott insulator wires, vanadium dioxide and vanadium sesquioxide, were prepared on the piezoelectric LiNbO3 substrates. Coupling of acoustic waves propagating in LiNbO3 with free carriers in vanadium oxide gives rise to the acoustoelectric effect that manifests itself as the generation of direct electric current by the acoustic wave. According to a phenomenological model, the value of the effect strongly depends on the wires conductivity, which, for the vanadium-oxide films, changes by a few orders of magnitude. We demonstrated that this yields a significant enhancement of the direct current (DC) current generated in the wires at the metal–insulator transition temperatures. The sign of the generated DC voltage is different for excitations by surface and bulk acoustic wave modes, which may happen due to reverse wave propagation at the substrate surface. For each resonance mode, polarities of the generated DC signal are the same in both wires, despite the signs of charge carriers being different for these materials. It was shown that two complementary techniques (acoustoelectric and Hall effect measurements) yield opposite signs of charge carriers in VO2.

Electrophysical properties of vanadium oxide films deposited by reactive magnetron sputtering

The aim of this work was to study the effect of the gas composition during sputtering on the electrophysical properties of vanadium oxide films deposited by pulsed reactive magnetron sputtering of a vanadium target in an Ar/O2 medium of working gases.The dependences of the magnetron discharge voltage, deposition rate, resistivity, temperature coefficient of resistance (TCR), and the band gap of vanadium oxide films on the oxygen concentration in the gas mixture are obtained. It was found that amorphous films of vanadium oxide are formed during reactive magnetron sputtering. It is shown that the properties of the deposited vanadium oxide films have a strong dependence on the oxygen concentration in the Ar/O2 gas mixture, which is associated with the formation of a mixture of various intermediate vanadium oxides in the film. It was found that from the point of view of using vanadium oxide films as thermosensitive layers of microbolometers, the films must be deposited at oxygen concentrations in the gas mixture of 17 to 25 %. At the given oxygen concentrations without heating the substrates, vanadium oxide films with a resistivity (0.6–4.0)·10-2 Ohm·m, TCR 2.2–2.3%/°C and a band gap for direct transitions of 3.7–3.78 eV. The obtained characteristics make it possible to use these films as thermosensitive layers of microbolometers.

Effect of source temperature on phase and metal–insulator transition temperature of vanadium oxide films grown by atomic layer deposition**Project supported by the National Natural Science Foundation of China (Grant Nos. 11674038, 61674021, 61704011, and 61904017), the Developing Project of Science and Technology of Jilin Province, China (Grant Nos. 20170520118JH and 20160520027JH), and the Youth Foundation of Changchun University of Science

Vanadium oxide films were grown by atomic layer deposition using the tetrakis[ethylmethylamino] vanadium as the vanadium precursor and H2O as the oxide source. The effect of the source temperature on the quality of vanadium oxide films and valence state was investigated. The crystallinity, surface morphology, film thickness, and photoelectric properties of the films were characterized by x-ray diffraction, atomic force microscope, scanning electron microscope, I–V characteristics curves, and UV–visible spectrophotometer. By varying the source temperature, the content of V6O11, VO2, and V6O13 in the vanadium oxide film increased, that is, as the temperature increased, the average oxidation state generally decreased to a lower value, which is attributed to the rising of the vapor pressure and the change of the ionization degree for organometallics. Meanwhile, the root-mean-square roughness decreased and the metal–insulator transition temperature reduced. Our study is great significance for the fabrication of vanadium oxide films by atomic layer deposition.

Metal-Organic Decomposition-Mediated Nanoparticulate Vanadium Oxide Hole Transporting Buffer Layer for Polymer Bulk-Heterojunction Solar Cells

In this study, a solution-processable compact vanadium oxide (V2O5) film with a globular nanoparticulate structure is introduced to the hole transport layer (HTL) of polymer bulk-heterojunction based solar cells comprised of PTB7:PC70BM by using a facile metal-organic decomposition method to replace the conventionally utilized poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). For this, a biocompatible structure-determining agent, polyethylene glycol (PEG, Mn 300), is used as an additive in the precursor to form the nanoparticulate compact V2O5 (hereafter referred to as NP-V2O5) film, which possesses an outstandingly smooth surface morphology. The introduction of NP-V2O5 HTL via the solution process with a neutral pH condition successfully improved the stability by preventing the decomposition of indium tin oxide (ITO) glass and the penetration of heavy-metal components and moisture, which are considered as the crucial drawbacks of using PEDOT:PSS. Over 1440 h (60 days) of the stability test, an organic solar cell (OSC) with NP-V2O5 showed a significant durability, maintaining 82% of its initial power conversion efficiency (PCE), whereas an OSC with PEDOT:PSS maintained 51% of its initial PCE. Furthermore, due to the positive effects of the modified surface properties of NP-V2O5, the PCE was slightly enhanced from 7.47% to 7.89% with a significant improvement in the short-circuit current density and fill factor.

Atomic Layer Deposition of Vanadium Oxide as Hole‐Selective Contact for Crystalline Silicon Solar Cells

AbstractHigh carrier recombination loss at the contact regions has become the dominant factor limiting the power conversion efficiency (PCE) of crystalline silicon (c‐Si) solar cells. Dopant‐free carrier‐selective contacts are being intensively developed to overcome this challenge. In this work, vanadium oxide (VOx) deposited by atomic layer deposition (ALD) is investigated and optimized as a potential hole‐selective contact for c‐Si solar cells. ALD VOx films are demonstrated to simultaneously offer a good surface passivation and an acceptable contact resistivity (ρc) on c‐Si, achieving a best contact recombination current density (J0) of ≈40 fA cm−2 and a minimum ρc of ≈95 mΩ.cm2. Combined with a high work function of 6.0 eV, ALD VOx films are proven to be an effective hole‐selective contact on c‐Si. By the implementation of hole‐selective VOx contact, the state‐of‐the‐art PCE of 21.6% on n‐type c‐Si solar cells with a high stability is demonstarted. These results demonstrate the high potential of ALD VOx as a stable hole‐transport layer for photovoltaic devices, with applications beyond c‐Si, such as perovskite and organic solar cells.

Thermochromatic vanadium dioxide (VO2) thin films synthesized by atomic layer deposition and post-treatments

As a thermochromatic material showing metal–insulator transition behavior below 70 ℃, crystalline films of vanadium dioxide (VO2) have potential applications in many fields. In this work, amorphous vanadium oxide (VxOy) films were deposited on glass substrates by atomic layer deposition (ALD). The film growth mechanism and growth rate were studied by quartz crystal micro balance, and a constant growth rate of about 0.5 Å/cycle was obtained. X-ray diffraction and X-ray photoelectron spectroscopy characterizations demonstrated that the as-deposited ALD VxOy film was amorphous with mostly V4+ oxidation state. Crystalline V2O5 films were prepared by annealing the as-deposited ALD VxOy films at 300 ℃ under O2 flow. Subsequently, different annealing methods and conditions were tested for transforming V2O5 films to VO2 films. It turned out that high purity crystalline VO2 films could be obtained by heating the V2O5 films in an oxidizing environment at a reduced pressure. Results of transmittance measurements revealed that these crystalline VO2 films had excellent thermochromatic properties with a reversible change in transmittance at about 65 ℃.

Thin Films of Nanocrystalline Vanadium Dioxide: Modification of the Properties, and Electrical Switching

Vanadium oxide films have been fabricated by the acetylacetonate and triethoxy vanadyl sol-gel methods on silicon substrates, as well as by magnetron sputtering on glass-ceramic substrates. Additional annealing in reducing atmosphere results in formation of vanadium dioxide or mixed phases with a VO2 predominance. The obtained films demonstrate the metal-insulator transition and electrical switching. In the films produced from triethoxy vanadyl, the peculiarities of electrical properties are related to the size effect, heterophase character of vanadium oxide films, and different types of charge carriers in the bulk of nanocrystallites and on their surfaces. Also, the effect of doping with hydrogen by means of plasma-immersion ion implantation on the properties of vanadium dioxide is explored. It is shown that the transition parameters in VO2 thin films depend on the hydrogen implantation dose. At doses exceeding a certain threshold value, the films are metallized, and the phase transition no longer occurs.

Synthesis and Electrical Resistance Property of Vanadium Oxide Thin Films by DC Magnetron Sputtering

Thin films of vanadium oxide were synthesized by DC magnetron sputtering on a glass slide. Process parameters including temperature, argon and oxygen ratio (Ar:O2) and base pressure were investigated. It was found that the control of sputtering base pressure is very important in realizing the pure VO2 (M) thin films. These thin films have grown at low base pressure of 5x10-6 mbar. The results of the valence electron analysis show the outstanding phase of VO2 and a small amount of V2O5 and V2O3 phases. The hysteresis loop of electrical resistance property of the VO2 thin films illustrated the quality of film, which is related to metal-insulator transition phase (MIT) at the transition temperature (Tc) of approximately 63°C. The sample was tested for its electrical properties by a four-probe setup with a temperature controller during heating and cooling cycles.

Spectroelectrochemistry in the investigation of sol–gel electrochromic V2O5 films

Spectroelectrochemistry can offer important insights into the performance of materials that change properties during electrochemical treatment. The benefits of spectroelectrochemical measurements are shown through the example of an electrochromic sol–gel vanadium(V) oxide (V2O5) film. In situ UV-visible absorbance spectroelectrochemistry is indispensable to reveal the optical modulation, which reached about 28% during the first coloration in the unsafe potential range, but then diminished to around 12% during the following cycles. Ex situ IR and Raman spectroscopy were used to probe the structural modifications that occur during the intercalation of small ions into the orthorhombic structure of the crystalline V2O5 film. In particular, infrared reflection–absorption (IR RA) and IR absorption spectroelectrochemistry confirmed the red shift of the vanadyl band and the disappearance of the bridging oxygen band due to the puckering of layers of VO5 square pyramids following intercalation. In addition, Raman spectra reveal the broad and low-intensity bands of the V2O5 film after it has been coloured in the unsafe potential range. Electron energy-loss spectroscopy (EELS) revealed the presence of vanadium in oxidation states V4+, V4+/V5+, and V5+ on different positions on the bleached film.

In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VO x.

Resistive switching crossbar architecture is highly desired in the field of digital memories due to low cost and high-density benefits. Different materials show variability in resistive switching properties due to the intrinsic nature of the material used, leading to discrepancies in the field because of underlying operation mechanisms. This highlights a need for a reliable technique to understand mechanisms using nanostructural observations. This protocol explains a detailed process and methodology of insitu nanostructural analysis as a result of electrical biasing using transmission electron microscopy (TEM). It provides visual and reliable evidence of underlying nanostructural changes in real time memory operations. Also included is the methodology of fabrication and electrical characterizations for asymmetric crossbar structures incorporating amorphous vanadium oxide. The protocol explained here for vanadium oxide films can be easily extended to any other materials in a metal-dielectric-metal sandwiched structure. Resistive switching crossbars are predicted to serve the programmable logic and neuromorphic circuits for next-generation memory devices, given the understanding of the operation mechanisms. This protocol reveals the switching mechanism in a reliable, timely, and cost-effective way in any type of resistive switching materials, and thereby predicts the device's applicability.

Reaction dynamics of metal/oxide catalysts: Methanol oxidation at vanadium oxide films on Rh(1 1 1) from UHV to 10−2 mbar

Recent advances in in situ microscopy allow to follow the reaction dynamics during a catalytic surface reaction from ultra-high vacuum to 0.1 mbar, thus bridging a large part of the pressure gap. Submonolayer vanadium oxide films on Rh(1 1 1) have been studied during catalytic methanol oxidation in situ with spatially resolving imaging techniques. At 10−6–10−4 mbar VOx condenses into macroscopic circular islands that exhibit a substructure, consisting of a reduced island core and an oxidized outer ring. This substructure arises due to an oxygen gradient inside the VOx islands, which results in different coexisting 2D-phases of VOx on Rh(1 1 1). This substructure is also responsible for a “breathing-like” oscillatory expansion and contraction that the islands undergo under stationary conditions. Using density functional theory, the 2D-phase diagram of VOx on Rh(1 1 1) has been computed. The oscillatory behavior can be understood as a periodic phase transition between two 2D phases of VOx. With a newly developed near ambient pressure – low-energy electron microscope, it was shown that VOx islands disintegrate at 10−2mbar, resulting in turbulent dynamics.

Performance improvement of Y-doped VOx microbolometers with nanomesh antireflection layer.

In the study, the yttrium (Y)-doped vanadium oxide (VOx:Y) films used as the sensitive layers of microbolometers were deposited using a radio frequency magnetron co-sputtering system. The temperature coefficient of resistance (TCR) of the VOx:Y films was enhanced from -1.88%/°C to -2.85%/°C in comparison with that of the VOx films. To further improve the performance of microbolometers, the nanomesh antireflection layer was placed on the top surface of the microbolometers to reduce the infrared reflection. The responsivity, thermal time constant, thermal conductivity, absorptance, and detectivity of the VOx:Y microbolometers with nanomesh antireflection layer were 931.89 ± 48 kV/W, 4.48 ms, 6.19×10-8 W/K, 74.41% and 2.20×108 cmHz0.5W-1, respectively.